A sintered product referred to herein as a cermet contains, as a hard phase component, a carbide, a nitride, or a carbonitride of Ti which serves as an essential metal component and a Group (4a, 5a, 6a) metal other than Ti, i.e., V, Cr, Zr, Nb, Mo, Hf, Ta, or W. Such materials have become a mainstream material for cutting tools due to their excellent properties, such as excellent plastic deformation resistance, relative to those of conventional super-hard alloys. In recent years, in order to further enhance fracture resistance, thermal shock resistance, and plastic deformation resistance, it has been proposed to incorporate, Ti carbide into the hard phase, of Ti nitride or Ti carbonitride having high toughness and thermal conductivity, (see, for example, Japanese Patent Publication (kokoku) No. 56-51201.
Furthermore, the Japanese Patent Application Laid-Open (kokai) No. 8-24690 discloses a microcrystalline structure which can enhance wear resistance and fracture resistance of the aforementioned Ti nitride-containing cermet. Namely, there is disclosed a core-shell structure in which the central portion (mainly formed of Ti carbonitride) of the core- containing hard phase of the cermet is formed of rounded block-shaped grains primarily formed of (Ti, M)CN and having an aspect ratio of 1.4 or less. There is also disclosed the structure in which the core-containing hard phase is formed such that a Ti carbonitride phase accounts for 90% or less by volume of the core-containing hard phase. This microcrystalline structure improves fracture resistance of aa tool through an increase in strength, while maintaining wear resistance thereof.
For enhancing the strength and hardness of the cermet, tools which are disclosed in the aforementioned publication, it is advantageous to form the central portion of the core-containing hard phase into a rounded-block shape in the microcrystalline structure. However, toughness in turn decreases, with the result that the thermal crack resistance of the substrate decreases. Thus, the problem of poor service life arises in applications that require high thermal crack resistance, such as intermittent cutting of steel.
Accordingly, an object of the present invention is to provide a cermet tool having excellent thermal crack resistance and a long service life even when the tool is used for intermittent cutting such as intermittent cutting of steel, while maintaining plastic deformation resistance and wear resistance that are advantageous points of cermets and to provide a method for manufacturing the same.